Marine propulsion systems generally operate to move a vessel through the water by producing an accelerated column of water. The column of water, known as the slipstream, provides a thrust against the propeller, nozzle or other propulsive device to push the vessel through the water.
The present invention, generally, provides an apparatus for increasing the thrust of a marine propulsion system without increasing the load on the engine.
More particularly, the present invention provides an apparatus for increasing the thrust of a conventional marine propulsion system by providing a blanket or layer of air encircling a slipstream produced by a propulsion system. According to a preferred embodiment, the invention comprises a cowling for a conventional marine propeller that radially encloses the propeller and extends downstream of the propeller to encircle the slipstream of the propeller. An inner part of the cowling is shaped as a section of a cone, with the propeller disposed in a front end of the cone section, and the cone section axially converging (or narrowing) in the rearward direction. An outer part of the cowling is tubular shaped and is longer than the inner part. A front end of the outer part is joined with a front end of the inner part so that the inner and outer parts form a rearward opening annular chamber. Means for providing air to the annular chamber is included to form a blanket of air around the propeller slipstream.
The means for providing air comprises at least a conduit leading engine exhaust to the annular chamber. Additional means includes a duct to lead ambient air to the annular chamber. Movement of the slipstream causes a vacuum in the annular chamber that draws air through the ambient air duct. Other suitable means may also be provided.
The invention is described in terms of a conventional propeller, however, it is understood that any apparatus which produces an accelerated column of water to move a vessel could be substituted with advantageous results in the invention, for example, a bow or stern thruster on the side of a vessel used for positioning the vessel for docking.